Fluids, such as natural gas, are transported, distributed, and sold to end-users (e.g., consumers) through a distribution system. For example, a furnace in an industrial plant may receive natural gas through a gas line. The gas consumption rate by the end user is known to vary over time.
A gas meter is generally installed on the gas line to measure the amount of gas consumed by the user. One type of gas meter is a rotary (mechanical) gas meter. Gas is typically sold by price per volume and is billed in price (e.g., $)/unit volume at base reference conditions of pressure and temperature as per applicable standards. Proper gas billing, as well as other functions, depends on accurate gas metering. Such conventional gas meters which measure the volume of flowing gas by mechanical means require correction because they do not take into account the effect of pressure (P), temperature (T) and super compressibility (Z) on the volume so that the measured volume lacks accuracy. P, T and Z corrections are applicable even for non-mechanical flow meters such as ultrasonic flow meters.
Gas meters are generally communicably coupled to a gas data downloading device for downloading consumption and gas meter readings generally being uncorrected gas volume data (UGVD), and the state variables (P, T) received from the gas meters. In the case of EVCs, each gas meter will be coupled to an EVC, and a separate dedicated EVC is generally provided at each service location to service a plurality of gas meters.
Most gas meters whether electronic or mechanical provide a pulsed output having a pulse count that corresponds to a particular UGVD generally referred to as an “uncorrected volume”. The gas density is recognized to change as a function P and T (as well as for composition changes for gas mixtures such as natural gas), and variations in these variables can result in differences in energy content for like volumes of gas. Hence the UGVD is generally compensated for P, T (and if applicable and available the composition) variations and is represented as CGVD at standard conditions of P and T. The output data from the primary gas meter is also referred to as “interval data”. Some gas meters have a local memory which enables the capability to provide accumulated gas meter readings for a particular interval of time, such as corresponding to a day, week or a month.
The EVC generally performs the P and T (and optionally for composition) correction to generate CGVD from the UGVD received from the gas meters or directly from gas pipe. The EVC has a local memory that enables the capability to store gas meter data for a particular duration, accumulated gas meter data (UGVD and CGVD) from a start and end time along with time stamps (typically time of use, ToU), and the EVC generates alarms in case of a detected error condition. Each EVC in the system will generally have a serial number (S/N) to enable a unique identification by the associated Meter Data Management (MDM) system.